Neutralisation Reactions

pH

The diagram below shows the pH scale, which goes from pH 1 to 14.

The pH of a solution can be measured using an indicator or a pH probe.

An indicator is a dye that turns a specific colour on the pH scale, depending on the pH of the solution. For example, if the indicator shows a colour of blue/purple, this shows that the solution is alkali.

  • On the left, between pH 0 – 6 are the acids – This ranges from dark red (which is a strong acid), all the way through to yellow (which is a weak acid
  • In the middle, at pH 7, you get neutral substances – This is where you find pure water
  • And on the right, between pH 8 – 14 are the alkalis – This ranges from dark green (weak alkalis) to purple (strong alkalis)

A pH probe is placed into the solution and then a number is presented which shows the acidity of the solution. If the number is 2, this shows that the solution is an acid. This method is more accurate than using an indicator as it shows numerical data.

If you add the right amount of an acid to an alkali, this will result in a neutral solution. And the same is in reverse, if you add the right amount of an alkali to an acid solution, this will result in a neutral solution.

This means if we had something from the left side of the pH scale (an acid) and added the right volume of something from the ride side of the pH scale (an acid), we would eventually form a substance in the middle. We would form a neutral solution at pH7.

We can tell the solution has reached a neutral state when the solution turns green.

Neutralisation

Neutralisation is the name of the reaction when acids react with bases

Acid + base ⮕ salt + water

Another way this equation can be written is with the ions:

H+ + OH ⮕ H₂O

You may be wondering why we’re using the word base instead of alkali. This is because, a base that dissolves in water is called an alkali.

If a base is insoluble, for example calcium hydroxide, it can’t dissolve in water. Which means it can’t form an alkali. However, if you reacted it with an acid, it would still form salt + water.

If a base is soluble, for example sodium hydroxide, it can dissolve in water and can therefore make an alkali. In this case, we can use the equation

  • Acid + base ⮕ salt + water

Let’s look at the example below:

In this example, hydrochloric acid (HCl) is reacting with sodium hydroxide (NaOH) to form Sodium chloride (NaCl) and water (H2O).

Remember!

In this case we have…

  • Hydrochloric acid – Which is the acid
  • Sodium hydroxide – Which is the base (and also alkali as it is a soluble base)

Forming

  • Sodium chloride – Which is the salt
  • Water

To get the name of the salt produced, you get the first part of the alkali (reactant) as the first word. This is ‘sodium’. The second part of the salt depends on the acid used in the reaction. As we used hydrochloric acid, the second part of the name will be ‘chloride’, This will form sodium chloride. Making the equation:

Hydrochloric acid + Sodium hydroxide ⮕ Sodium chloride + Water

Or more specifically

HCl + NaOH ⮕ NaCl + H2O